1. Technical Field
The invention relates to reactors for performing radio frequency (RF) plasma chemical vapor deposition (CVD) and sputter etch processes and particularly to such reactors for performing both processes simultaneously.
2. Background Art
CVD formation of a thin silicon dioxide film on an integrated circuit structure having small (0.5 .mu.m or less) features with high aspect ratios (i.e., a large value of the ratio of channel depth to channel width, e.g., greater than two) is nearly impossible to accomplish without formation of voids between the metal lines. As shown in FIG. 1A, in depositing a dielectric material 10 on a device having a very narrow channel 12 (i.e., an aspect ratio greater than 2) separating two metal lines 14a, 14b, relatively little of the dielectric material 10 reaches the bottom of the channel 12, leaving a void 15. This is because dielectric material 10 is deposited more quickly at the corners 16 of the metal lines 14 than elsewhere along the vertical walls of the metal lines 14, thus at least nearly sealing off the bottom of the channel 12 during the deposition process. A solution to this problem is to simultaneously etch the dielectric material 10 from the corners while depositing using an RF sputter etch process that uses ions impinging vertically on the surface, thus preventing pinching off of the channel 12. This process can be used for spaces with aspect ratios greater than two, unlike currently-used sequential deposition and sputtering which fails below 0.5 .mu.m.
As illustrated in the graph of FIG. 1B, an RF sputter etch process has a maximum etch rate for surfaces disposed at a 45.degree. angle relative to the incoming ions. By directing the ions to impinge in a perpendicular direction relative to the wafer surface, the sputter etch process quickly etches angled surfaces formed by the simultaneous deposition process (such as dielectric surfaces formed over the corners 16) and etches other surfaces (i.e., horizontal and vertical surfaces) much more slowly, thus preventing the blockage of the channel 12 and formation of the void 15 shown in FIG. 1A. This permits deposition of dielectric material preferentially at the bottom of the channel 12 and on top of the lines 14, relative to the side walls and corners 16, as illustrated in FIG. 1C.
In order to accomplish the foregoing, the RF plasma sputter etch rate near the corners 16 must be on the order of the deposition rate. High plasma density is required to meet the requirement of high sputtering rate (production throughput) without electrical damage to the semiconductor devices. In order to achieve such a sputter etch rate across an entire wafer (such as an eight inch Silicon wafer), the plasma ion density must be sufficiently high and uniform across the entire wafer. Such uniformity is readily accomplished using a plasma consisting almost entirely of argon ions. However, it will be remembered that the sputter etch process desired here is ancillary to a CVD process requiring species other than argon to be present. Specifically, in a CVD process employing silane (SiH.sub.4) in which the dielectric material 10 is SiO.sub.2, oxygen must be present in significant quantities, the oxygen being ionized in the plasma. The oxygen ions have a relatively short lifetime and are highly susceptible to quenching. It is very difficult to attain a dense and very uniform distribution of oxygen ions across the wafer surface, particularly 8-inch diameter wafers of the type now currently in use.
While the plasma may be generated with electron cyclotron resonance (ECR), ECR apparatus has limited commercial attractiveness due to design complexity, size and cost. Moreover, since the plasma is generated remotely from the wafer, scaling the ECR reactor up to accommodate an 8-inch wafer diameter is difficult and requires simultaneous use of complex magnetic fields.
Application of inductively coupled plasmas to high-rate sputter etching in CVD systems is disclosed in application Ser. No. 07/941,507 filed Sep. 8, 1992 by Collins et al. entitled "Plasma Reactor Using Electromagnetic RF Coupling and Processes" and assigned to the assignee of the present application, the disclosure of which is hereby incorporated by reference in its entirety into the present specification. An earlier version of this work is described in European patent publication EP 0,520,519 A1. As described therein, one advantage of inductively coupled plasmas over capacitively coupled plasmas is that the inductively coupled plasma is generated with a much smaller bias voltage on the wafer (reducing the likelihood of damage thereto) even in the presence of a greater plasma density. In the silicon oxide deposition disclosed in the referenced patent application, silane, mostly unionized, provides the silicon and a gaseous oxygen species provides the oxygen for the formation of silicon dioxide by CVD. Argon ions accelerated across the sheath adjacent the wafer are used for sputter etching.
FIG. 2 illustrates a CVD vacuum chamber 20 and RP antenna 22 for generating an inductively coupled plasma of the general type disclosed in the above-referenced application, although that particular chamber had a top-hat shape. The RF antenna 22 is a coiled conductor wound as a solenoid around the cylindrical vertical side wall 24 of the vacuum chamber 20. The source chamber wall adjacent the coil antenna is an insulator while the ceiling 26 and the process chamber walls are preferably grounded, the flat ceiling 26 functioning as a grounded electrode.
The cylindrical coil of the referenced application non-resonantly couples the RF energy in the coil antenna into the plasma source region via an induced azimuthal electric field. Even in free space, the electric field falls to zero at the center of the chamber. When a plasma is present, the electric field falls off even more quickly away from the chamber walls. The electric field accelerates electrons present in the plasma, which then further ionize atoms into ions or break up molecules into atoms or radicals. Because the coupling is not tuned to a plasma resonance, the coupling is much less dependent on frequency, pressure and local geometries. The plasma source region is designed to be spaced apart from the wafers, and the ions and atoms or radicals generated in the source region diffuse to the wafer.
The chamber of the above-referenced application is primarily designed for etching at relatively low chamber pressures, at which the electrons have mean free paths on the order of centimeters. Therefore, we believe the electrons, even though primarily generated near the chamber walls, diffuse toward the center and tend to homogenize the plasma across a significant diameter of the source region. As a result, the diffusion of ions and atoms or radicals to the wafer tend to be relatively uniform across the wafer.
We believe the reactor of the above-referenced application has a problem when it is used for CVD deposition and sputter etching, particularly involving oxygen. For CVD, the chamber pressure tends to be somewhat higher, reducing the electron mean free path and resulting in a nonuniform plasma density with the peak density occurring in an outer annulus of the plasma. Furthermore, oxygen ions or radicals are subject to many recombination paths so that their diffusion lengths are relatively limited. Thus, the wafer center is farther from the plasma source region than the wafer edges, and the oxygen ion and radical density is less near the center of the wafer 28 than it is at the edges thereof, as illustrated in the solid line curve of ion density of FIG. 3. The lack of oxygen ions near the wafer center reduces the sputter etch rate relative to the CVD deposition rate, leading to formation of the void 15 as illustrated in FIG. 1A in spaces or channels near the wafer center (e.g., the channel 12 of FIG. 1A), while spaces near the wafer periphery have the desired ratio between sputtering and deposition rates.
One possible solution would be to raise the height of the ceiling 26 and to increase the axial height of the antenna 22 above the wafer. (For argon only, the ion distribution for this taller source would be virtually uniform in accordance with the dashed-line curve of FIG. 3.) However, such a height increase is impractical because the larger volume makes cleaning of the system more difficult. Another possible solution would be to operate the source region at a very low pressure (below 1 milliTorr), at which the oxygen ion density is quite uniform and ion distribution may not be as severe a problem, depending upon the distance of the wafer to the top electrode. However, maintaining such a hard vacuum requires an impractically large pump size, and so a relatively lower vacuum (higher pressure) between 1 and 30 milliTorr is needed for commercial viability.
Some of these problems are addressed by Ogle in U.S. Pat. No. 4,948,458 by the use of a planar spiral coil antenna placed on a flat dielectric chamber top. This is sometimes called a pancake coil. Such a design is claimed to create a uniform plasma source region adjacent the top of the chamber, thus providing uniform ion and radical diffusion to the wafer.
However, we believe the pancake coil to have drawbacks. Its planar configuration suggests that a significant part of its RF power coupling into the chamber is capacitive coupling, that is, it uses electric fields set up by charge accumulation in the antenna structure rather than electric fields induced by current flow through the antenna, as is the case with inductive coupling. Capacitive coupling generally creates very high electric fields, which in turn create high-energy electrons that are deleterious in a semiconductor reactor. In contrast, the predominantly inductive coupling of the above-referenced application of Collins et al. produces lower electric fields and lower electron energies.
Accordingly, there is a need to uniformly distribute oxygen ions in high density inductively coupled plasmas between 1 and 30 milliTorr across large (8-inch) wafers in order to maintain uniform oxygen sputter or etch rates on the order of 1000 Angstroms per minute.
Another problem is that silane emitted from the gas outlets 30 in the sides of the vacuum chamber 20 diffuses equally in all directions, not just toward the wafer 28. Since the silane and oxygen gases react together spontaneously, and since the chamber walls are closer to the gas outlets 30 than most of the wafer 28 (particularly for larger diameter wafers), deposition of SiO.sub.2 over all interior surfaces of the vacuum chamber 20 is greater than that on the wafer 28. This means that the reactor must be periodically removed from productive activity and the SiO coating removed from the interior surfaces, a significant disadvantage.
Thus, there is a need for a reactor which deposits less CVD residue (e.g., SiO.sub.2) on the interior chamber surfaces and which therefore requires less frequent cleaning.